The present invention is generally concerned with mobile radiocommunication systems.
The present invention is more particularly concerned with power control techniques used in such systems to improve performances (in terms of quality of service, of capacity, . . . etc.).
The present invention is in particular applicable to mobile radiocommunication systems of CDMA (Code Division Multiple Access) type. In particular, the present invention is applicable to UMTS (Universal Mobile Telecommunication System).
The CDMA is a multiple access technique which makes it possible for several users to be simultaneously active, using different spreading codes.
All that follows is valid for both downlink (link from BTS (Base Transceiver Station) to MS (Mobile Station)) and uplink (link from MS to BTS), but in order to simplify the description, only the downlink case will first be considered.
The quality of the link from a BTS to a MS depends on the ratio of the received signal power and the interference power at the MS (SIR: signal-to-interference ratio). When the SIR of one MS is low, or equivalency when the interference power is much larger than its power, its performance dramatically decreases. Therefore, in order to optimize the performance of a CDMA system, some algorithms are usually used in order to keep the SIR of each MS as close as possible to the target SIR at the receiver, like the inner loop power control algorithm.
The principle of the inner loop power control algorithm is that the MS periodically estimates the SIR of the received signal from the BTS, and compares this SIR to the target SIR (SIRtarget). If this estimated SIR is lower than the target SIR, the MS sends a command to the BTS for the BTS to increase its transmit power. Otherwise, the MS sends a command to the BTS for the BTS to decrease its transmit power. The target SIR is chosen by the MS (or BTS) in function of the required quality of service.
Additionally, another and usually slower power control algorithm, namely outer loop power control algorithm, enables to choose the best value of the target SIR. The principle of this algorithm is to regularly evaluate the quality of the transmission (BER, BLER, . . . ) and to compare this quality with the required quality of service (for example BER of 10−3 for speech service, BLER of 0.1 for packet service, . . . ). If this quality is below the required quality of service, the target SIR is increased. Otherwise, the target SIR is decreased. This algorithm is usually slow, since the quality needs to be averaged over several frames in order to have a reliable estimate. Of course many variants of this basic algorithm exist.
In some situations, the target SIR may change significantly during the transmission. For example, this is the case when the spreading factor of the physical data channel changes. Indeed, the lowest the spreading factor of this channel, the largest the required transmit power. The spreading factor can change frequently in variable rate services such as packet service. Indeed, if the spreading factor changes, the target SIR will vary much (in the ratio of the spreading factor variation). It is also the case if the MS requires to change of service, since each service has a different target SIR.
Another example is the compressed mode. In an inter-frequency hard handover, the mobile needs to make measurements on a frequency different from the frequency used for the downlink transmission. Thus, the base station needs to stop its transmission towards the concerned mobile, in order to allow this mobile to make measurements on this other frequency. In the UMTS standard, this is known as downlink compressed mode (i.e. the downlink transmission is temporarily stopped). Uplink compressed mode is also possible to make measurements on frequencies that are close to the uplink frequency. The periods where transmission is stopped are usually called transmission gaps, and the frames including transmission gaps are usually called compressed frames. Besides, to compensate for the transmission gaps, the transmission rate has to be correspondingly increased. Therefore, during compressed mode, since the inner loop power control is regularly stopped, and since the transmission rate is correspondingly increased, the target SIR needs to be larger to reach the same quality of service than during non-compressed, or normal, mode.
Because the outer-loop power control algorithm is usually a slow process, the target SIR will not change immediately and the transmission quality will be degraded during several frames. In extreme cases, this could cause the lost of the call.
Moreover, in the case of compressed mode, the target SIR needs to be changed only at certain fixed time to enable the mobile to perform measurements and then the target SIR needs to be changed back to the previous value. The outer-loop power control algorithm will not be able to track such quick variations of SIR.
In European patent application n° 99401766.3 filed on Jul. 13, 1999 by Applicant, a solution has been proposed to solve this problem. Briefly, the basic idea in this prior patent application is to anticipate the target SIR variation, i.e. to apply an expected variation, or offset, in an anticipated way, to the target SIR. This target SIR variation may be signaled from the transmitter to the receiver for a given transmission direction; for example, for downlink transmission, it may be signalled by the network to the MS or UE (User Equipment).
According to another idea in this prior patent application, in order to keep the signaling as low as possible, the target SIR increase due to the increased instantaneous bit rate and the target SIR increase due to degraded performances in compressed frames (i.e. due to transmission gaps) may be separated. For example, when the transmission rate increase in compressed mode is obtained by spreading factor reduction, this may be written:ΔSIR=10log(RCF/R)+δSIR where R is the instantaneous net bit rate before and after the compressed frame and RCF is the instantaneous net bit rate during the compressed frame (it being understood that the term “instantaneous bit rate” means that for a compressed frame, the time period used to calculate this rate is not the whole frame period but only the fraction of this frame period where data are transmitted); for example, 10log(RcF/R) is equal to 3 dB for UMTS, where the matching rate is the same for compressed and non compressed frames, when compressed mode by reducing the spreading factor by a factor of 2 is used.
Since the bit rate variation will be known by the UE, only the additional target SIR increase δSIR due to degraded performances during compressed frames may be signaled. The signaling overhead can be low if this variation is signaled with other compressed mode parameters (including transmission gap length (or period where transmission is stopped, periodicity, . . . ). For example, 2 bits could enable to signal the following values of δSIR:                −00: 0 dB        −01: 0.5 dB        −10: 1 dB        −11: 2 dB        
Alternatively, ΔSIR could be directly signaled, but a larger number of bits would be required.
The UE will have to increase the target SIR by ΔSIR just before the compressed frames (or just after the transmission gap of the compressed frames) and decrease it back by the same value just after the compressed frames. This target SIR variation is done additionally to the usual downlink outer-loop algorithm that will have to take it into account. The Node B may increase simultaneously its transmit power by the same amount before the compressed frame and decrease it just after the compressed frames in order for the downlink received SIR to be as quickly as possible close to this new target SIR.
According to another idea in this prior patent application, at least when the transmission gap is at the end of the compressed frame, the performances in recovery frames (frames following the compressed frames) can also be degraded because of the power control interruption during the transmission gap. Therefore, it would be also desirable to increase the target SIR in recovery frames and to signal this target SIR increase to the UE. Alternatively, the same value (δSIR) as for compressed frames could be used in order to decrease the required signaling.
Therefore, according to this prior patent application, by anticipating the target SIR variation during compressed frames and recovery frames, an efficient outer loop power control in compressed mode can be achieved, at least when said compressed mode is obtained by reducing the spreading factor.
Now, in the UMTS standard for example, two ways exist to perform compressed mode:                reducing the spreading factor in the compressed frame, enabling to increase the instantaneous bit rate and thus to stop the transmission during a few slots,        using puncturing (i.e. several bits obtained after channel coding are not transmitted, so that the same amount of information bits can be sent over a shorter period, knowing that the channel coding will still enable to decode all information bits).        
Compressed mode by puncturing has some particularities, which can be recalled by reference to the UMTS system for example.
One feature of UMTS is the possibility to transport multiple services on a same connection, i.e. multiple transport channels on a same physical channel. Such Transport Channels or TrCHs are separately processed according to a channel coding scheme (including error detecting, error correcting, rate matching, and interleaving) before being time-multiplexed to form a Coded Composite Transport Channel or CCTrCH to be mapped onto one or more physical channels. Processing according to this channel coding scheme is on a TTI (Transmission Time Interval) basis. In this channel coding scheme, rate matching includes the two techniques of puncturing and repeating; besides, an inter-frame interleaving is performed on the TTI length, or interleaving depth. Then each TTI is segmented into frames, and, after that, time-multiplexing and mapping on the physical channlel(s) are performed on a frame basis. Besides, each of the different transport channels TrCHi (i=1, . . . n) which are multiplexed to form a CCTrCH has its own TTI length, noted TTIi.
More information on these aspects of UMTS can be found in Technical Specification 3G TS25 212 V3.0.0 (1999-10).
Puncturing in compressed mode, which is included in rate matching, and which an be provided in addition to puncturing or repetition in normal mode, can either be performed on a frame basis or on a TTI basis.
If puncturing in compressed mode is performed on a frame basis, the above-recalled method according to the prior patent application still applies.
If puncturing in compressed mode is performed on a TTI basis, the transmission rate increase due to compressed mode applies to all frames of a TTI. Now, in the UMTS standard, TTI can be equal to 10, 20, 40, or 80 ms. Besides, as already mentioned, each of the different transport channels TrCHi (i=1, . . . n) which are multiplexed to form a CCTrCH has its own TTI length, noted TTIi. This is illustrated in FIG. 1, taking the example of three multiplexed transport channels noted TrCH1, TrCH2, TrCH3, and taking the example of TTI=40 ms for TrCH1, TTI=20 ms for TrCH2, TTI=10 ms for TrCH1, and a frame length equal to 10 ms. In this figure, the case of four consecutive frames sent on a physical channel is illustrated by way of example, and the case of a transmission gap TG overlapping two consecutive frames (in the circumstances the second and the third one of the four illustrated frames) is also illustrated by way of example.
In standardization proposal TSGR1#10(00)0086 presented at the 3GPP TSG-RAN Working Group 1 meeting #10 Beijing, China, Jan. 18-21, 2000, a modification of the above recalled method was presented for the case where the frames are compressed using puncturing and where puncturing is performed on a TTI basis.
According to this proposal of modification:
If there are “n” different TTI lengths in the CCTrCH (i.e. “n” transport channels multiplexed into the CCTrCH), then “n” separate DeltaSIR values (defined as coding gain degradation due to “too much” puncturing) DeltaSIR, i=1 . . . n, one for each TTI length, are signaled to the UE. These “n” DeltaSIR values should then be used in the following way for the outer loop power control.
For each frame the offset of the target SIR in compressed mode compared to target SIR in normal mode is:ΔSIRframe=max(ΔSIR1, . . . , ΔSIRn)where:ΔSIRi=ΔSIRi_compression+ΔSIRi_coding 
If there is no transmission gap within the current TTIims for the TTI length of TTIi (i.e. within the current TTI of the transport channel TrCHi which is multiplexed inside this frame, as may also be understood by referring to FIG. 1), then:ΔSIRi_compression=0ΔSIRi_coding=0
If there is a transmission gap within the current TTIims for the TTI length of TTIi, then:ΔSIRi_compression=10 log (Fi•N/(Fi•N−TGLFi)) ΔSIRi_coding=DeltaSIRi
Here Fi is the number of frames in the TTIi, TGLFi is the gap length in slots (either from one gap, or a sum of several gaps) within those Fi frames, and N is the number of slots per frame (N=15 in the UMTS standard).
This method (hereafter also referred to as second method) therefore requires additional signaling compared to the above-recalled one (hereafter also referred to as first method) according to the above-mentioned prior patent application. Indeed, the values DeltaSIRi are signaled for each value of “i”, i.e. for all possible values of TTI for the TrCHs multiplexed into the CCTrCH, therefore up to four values (the four possible values for TTI). Thus this second method does not make an efficient use of available radio resources, or needlessly contributes to a traffic increase in the network. Besides this second method increases the complexity, compared to the first one.
What would be desirable is a method which, notwithstanding the particularities of the compressed mode by puncturing, would not involve an increase in the amount of signaling compared to the first method, while still providing an efficient compensation for this type of compressed mode.
What would also be desirable is a method which, notwithstanding these particularities, would involve as few changes as possible compared to this first method, in order to be as far as possible applicable to both types of compressed modes (by puncturing and by reducing the spreading factor).
What would also be desirable is a method which would not involve a further increase in the amount of signalling and the complexity compared to the first method, while still providing an efficient compensation, in special cases of occurrence of compressed frames, such as when a transmission gap overlaps two consecutive frames.
In other words, there is a general need to simplify the signalling and the architecture of the equipments, while still providing an efficient compensation for outer-loop power control in compressed mode, in various types of compressed mode and/or various cases of occurrence of compressed frames.